Raytracing Subsurface Scattering + Two Sided Foliage - Bug?


i tried to create some materials with subsurface scattering and two sided foliages for RTX, but it seems, that either i am doing something wrong, or there is a bug with those two :frowning: After testing it in a brandnew scene withouth raytracing, the mats worked as i hopend, and as i could see in the manuals and several clips on youtube.
However, after duplicating this exact scene and activating RTX and all relevant stuff for it, the results for those materials differ from the non-RTX version. Without RTX you get lighter and darker areas, on partially shadowed areas, with RTX on, those areas are all uniform colored. What am i missing?

It’s not a bug but a ‘feature’ of RTX. Basically raytracing shadows work inverted to how the ‘old’ shadow maps are calculated hence it struggles with light conducting materials as we’ve got them atm. If you absolutely need this with RTX, the one working method is to have your Skylight cast raytracing shadows where your directional light is still set to CSM’s. Wish I had better news :frowning:

Thanks for the fast reply. And thats a real bummer, because i thought, Epic said, RTX is production ready. Is such a case not required for this? :frowning: Whatever, tried some stuff but got it not working. however, i found some strange behaviour, which seems to be a real bug, not only a weird feature.

If i watch those planes from a very flat angle, then at least for the two sided foliage, i can see though, sss mat still doesn´t work at all, regardless of CSMs.

Or is there an idiotproof manual/tutorial on how to get it working with RTX?

I filed a bug report a while ago, here’s the tracker:

As a workaround you can deactivate two sided material shadows via cvar, not ideal of course:
r.RayTracing.Shadows.EnableTwoSidedGeometry 0

I was also surprised that Epic declared ray tracing production ready, given the amount of bugs and shortcomings it still has.

Thanks for the workaround, that at least works for the two sided foliages. So until they fix it both subsurface scattering AND the foliages, i have to choose between getting the correct shadows from those foliages and NO shining through light, or getting the light shining through but losing the ability of getting shadows from the foliages projected on the ground and other objects. Have just noticed, how your leaves don´t create shadows anymore on the ground.

I also noticed, that the planes also get NOT reflected with their subsurface color (if you put something reflective behind them), they are always reflected with their normal color from the color input.

And the fix is just planned for 4.26, not as hotfix earlier :eek:

Same. RT shadows + SSS on foliage equals total garbage and yet apparently it’s no biggy.
Yuri from Epic commented it should be possible to make a hybrid solution where CSM’s still drive the SSS part on RT shadows but that comment alone shows how hardcore this issue really is and by that very nature I deem it unlikely we’ll see this fixed any time soon. 6 Votes on the tracker…that’s just surreal as this effects everyone.

I might just be an idiot but didn’t ray traced sss work in 4.23 already? This guy demonstrates sss with raytracing and rtgi enabled

Ray traced sss gets explained briefly in the docs on this page (just ctrl f and type sub surface)

SSS and twosided translucency are two completely different things. SSS simulates scattering of the light inside a surface with substantial depth where as twosided translucency is approximation of scattering light passing through very thin surfaces (leaves, sheets of paper, fabric).

The two sided foliage shader is working now with RT. Just make sure to enable transmission in the directional light settings.